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So basically my brother and I live in the same house and we always play Siege together. Normally it works. But Sometimes when my brother is play Siege, I can't connect to the server. 
Hello there, JamesCkw! Typically, playing online using the same Wi-Fi connection can potentially cause unexpected outlying issues regardless, thus it may be difficult to properly troubleshoot some errors in their entirety.
With that said, I suggest you give the steps found within this connectivity FAQ a try, in order to clear your connection to Siege's servers, and allow a smoother line of communication between the two.